The present invention relates to a method of measuring quantities that indicate the state of an electrochemical device and an apparatus for the same. More particularly, this invention relates to a method of measuring (a) properties of battery materials in initial state or in degraded state and (b) remaining charge/discharge capacity of the battery.
Electrochemical devices such as batteries, capacitors and electrochromic devices are widely used in electric and electronic apparatuses. Recently, in particular, batteries are attracting much attention. Because mobile electronics apparatuses such as notebook computers or video camcorders become popular, and energy consumption and the public concern about the environmental issues has increased.
While there are diverse types of batteries, they are classified into two categories: One is the primary battery that is not rechargeable and another is the secondary battery that is rechargeable.
Representative of the primary battery is alkali battery. There are also Leclanche battery, lithium battery, zinc-air battery and other types. The secondary batteries include lithium ion secondary battery, nickel-hydrogen battery, nickel-cadmium battery, lead-acid battery and fuel cell. Lithium ion secondary battery has been finding increasingly wider applications in recent years. Nickel-hydrogen storage battery is used for large-capacity power supply such as in electric automobiles.
In addition, there are physical cells such as solar cell and thermal battery.
Incessant research efforts have been made to develop new batteries or new component materials for batteries in order to improve storage capacity, energy density and cycle performance.
A work for the research and development of battery component materials is, for example, carried out as follows. First, promising materials to make high-capacity electrodes are synthesized. Electrochemical characteristics of a single electrode in an open cell made of these materials are measured, to evaluate the performance of these materials. Then a battery is built with materials having better characteristics for a positive or a negative electrode, and performance of this battery is measured in a laboratory test. At the same time, characteristics of materials to make other battery components such as an electrolyte and a separator are also examined. Using the materials that have been selected, a battery is built to evaluate the battery performance.
When the battery is used for an extended period of time, constituent materials of the battery degrade over time. Thus, it becomes necessary to analyze the state of the cell materials. Analysis of degraded battery materials has been previously done in such a procedure as shown in FIG. 14. First, the degraded battery is disassembled (step 501), and physical or physicochemical parameters of the constituent materials such as electrodes, electrolyte and the separator are measured (step 502), thereby to determine the properties of the constituent materials (step 503). Then, By comparing the determined properties with that of the materials, before assembled into the battery (step 504), degradation of the materials can be determined.
However, many of the steps in these measurements require skills and take time as long as several hours to several days.
Other measurement methods of degree of degradation of the battery are as follows:
(1) Measurement of internal impedance of the battery (Japanese Patent Publication (Laid-Open) Nos. 53-42327 and 61-170678) PA1 (2) Measurement of internal impedance of the battery by means of signals of different frequencies and processing of the measured values with a given equation (Japanese Patent Publication (Laid-Open) Nos. 8-43506 and 8-250159) PA1 (3) Measurement of electrical resistance of an active material that is a constituent element of the battery (Japanese Patent Publication (Laid-Open) No. 56-103875) PA1 (4) Comparison of voltage measured after discharging current for a predetermined period of time to a predetermined reference value (Japanese Patent Publication (Laid-Open) Nos. 59-48661, 3-95872, 8-254573, 8-55642 and 9-33620) PA1 (5) Counting the number of charge/discharge cycles (Japanese Patent Publication (Laid-Open) Nos. 5-74501 and 6-20724) PA1 (1) Measurement of change in specific gravity of the electrolyte PA1 (2) Integration of actual output current and input current (Current integration method) PA1 (3) Measurement of battery voltage under unloaded condition PA1 (4) Measurement of battery voltage under loaded condition PA1 (1) Measurement of voltage drop of the battery when overcharged: Detection of full charge by .DELTA.V PA1 (2) Detection of heat generated during overcharging with a thermistor, and detection of full-charge state by the value of dT/dt PA1 (a) a series of theoretical data of an electrical characteristic obtained by combining at least one of an electron transportation model of the electrode, an ion transportation model of the electrode, an ion conduction model of the ionic conductor and a model of the electrochemical reaction taking place in the interface between the electrode and the ionic conductor, with a potential model of the electrode, with PA1 (b) a series of measured data of the electrical characteristic of the electrochemical device. PA1 (a) a memory section for storing a series of theoretical data of an electrical characteristic obtained by combining at least one of the electron transportation model of the electrode, the ion transportation model of the electrode, the ion conduction model of the ionic conductor and a model of the electrochemical reaction that takes place in the interface between the electrode and the ionic conductor, with the potential model of the electrode; PA1 (b) an input section for entering a series of measured data of the electrical characteristic of the electrochemical device; PA1 (c) an arithmetic operation section for comparing the theoretical data stored in the memory section with the measured data entered through the input section; and PA1 (d) an output section for giving an output of the result of arithmetic operation from the arithmetic operation section.
However, these methods of measuring the degree of degradation of the battery are low in accuracy. The measurement accuracy becomes especially low after storage at a high temperature or repeated discharge/charge cycles.
It is also important to correctly measure the remaining capacity of the battery to increase the rate of the battery energy utilization. For example, sensing the full-charge state of a secondary battery is important to prevent overcharging of the battery in quick charging. Variety of measurement methods have begun to be widely used for primary batteries, secondary batteries and secondary battery chargers.
The following methods have been proposed for measuring the remaining capacity of batteries.
Full-charged state of a secondary battery during quick charging is sensed by the following methods.
However, measuring accuracy of these methods are low, especially after storage at a high temperature or repeated charge-discharge cycles.
An object of the present invention is to provide a method of and an apparatus for accurately measuring the material properties or remaining capacity of the electrochemical device without disassembling.